HR drift implications for Z2 training

Hi all. Recently purchased Training for the Uphill Athlete, did the Heart Rate Drift Test per the book club videos, and I’m wondering now what this means for the Z2 training ahead. From what I understand, the purpose of the Heart Rate Drift Test is to establish a HR at the top of our Z2. The test is dispositive because we would expect an increase in average heart rate between the first half hour of the test and second half of the test when we hold all other conditions constant (pace, terrain, etc). To the extent that we get the HR correct, the average HR in the first half of the test is the top of our Z2. This means that, for Z2 runs, if we run at the HR determined in the test or slightly below, we’ll get the desired effect of pushing our AeT  HR upwards or, if fit enough, lowering our pace for that HR.

Assuming I have all that right and have just summarized what’s stated elsewhere, why is it that for Z2 runs we do not keep our effort at a level that would follow this HR drift. For example, if my AeT HR is 155 bpm, why not increase our HR by a little less than 5% every half hour?

And, just to really help this all sink in if you don’t mind, what’s going on metabolically here? The heart is pumping faster to keep the same pace. More oxygen for more aerobic metabolism? Does that mean there’s a slow drain in glycogen stores and the body is changing HR to rely more on fat/aerobic metabolism? Am I way off :)?

Thanks in advance!

• This topic was modified 2 weeks, 3 days ago by LevityThroughElevation.